1,721,261 research outputs found
Linear features on side-scan sonar images: an algorithm for the correction of angular distortion
No full textThe distortion ellipse graphical method is used regularly by scientists for the correction of compressional effects caused in side-scan sonar images by variable ship speeds. An algorithm is derived for this correction for the calculation of the true angle of linear objects (e.g. sandwaves) in relation to the ship's track:where R is the ratio of the ship's speed to the optimal ship speed (i.e. for no distortion) and ? is the false angle between the ship's track and the linear object measured directly from the image (sonograph) (0° < ? < 90°). The algorithm is applicable for any combination of chart paper speed and width. The results obtained using this approach are considered to be more accurate than conventional graphical methods of analysis of analogue output. Interpretation of imagery for sedimentological investigations should be enhanced by the adoption of this procedure.<br/
Sediment resuspension on beaches: response to breaking waves
No full textField data of time-averaged suspended sediment concentration (ssc) are presented from three different UK beaches and a variety of wave conditions. The data are collated into a single analysis to examine the mechanisms associated with sand resuspension in the surf zone. Emphasis is placed upon the relative contribution of bottom and surface generated turbulence in controlling suspended sediment concentration distribution. Inside the surf zone, vortices induced by breaking waves and bores are the main mechanism for sediment resuspension; this process dominates over bottom boundary layer processes. Time-averaged reference concentration, defined as the ssc at 3.5 cm above the bed, does not correlate with bottom-induced shear stress or predicted ripple steepness. Both reference concentration and vertical distribution of sediment are controlled by the breaking wave (plunging, spilling) characteristics. Within the inner surf zone, hydraulic jumps (associated with strong offshore flows) are responsible for sediment resuspension. Vertical distribution of sediment is parameterised using a convective type time-averaged suspended sediment concentration profile. Reference concentration is highly correlated with wave breaking type, as defined by the local breaker parameter. The mixing parameter depends also on the wave breaking type and may be expressed as a fraction of the local water depth
The influence of clay on the threshold of movement of fine sandy beds
No full textThe erosion threshold (i.e. surface shear strength) of mixed sediment deposits has been tested under the action of unidirectional currents (mean critical speeds, 4 mm above the bed, ranging from 12.4 cm s?1 to 18.2 cm s?1) or simulated waves (mean critical periods ranging from 2.0 s to 6.7 s, for near-bed wave amplitudes of 0.28–0.57 m) using an oscillating tray in a rectangular recirculating flume. The sediment mixtures investigated consisted of angular fine-grained quartz sands (D50 = 152.5 and 215 ?m) and cohesive estuarine mud. The investigation has shown that the cohesive material increases generally the erosion threshold of sandy deposits. The rate of increase is smaller for mud contents < 30% (clay mineral content < 11–14%, by dry weight) and is attributed to an increase of the internal angle of friction. When the clay mineral content exceeds 11–14% by dry weight, then the sand particles are not in contact with each other. Consequently, the clay component controls the erodibility of the sediment mixture. Finally, a prolonged and intense stress history can increase considerably (by a factor of 1.3–1.7) the original erosion threshold value of these sediments
Laboratory investigations into wave period effects on sand bed erodibility, under the combined action of waves and currents
No full textLaboratory experimental data on sediment threshold conditions for fine to coarse sands, under the combined action of waves and currents are presented. Higher flows are needed to satisfy threshold under short period waves than for longer period waves. The combined threshold velocities indicate that under the shorter (5 s) period waves, the oscillatory and steady flows are independent of each other, but interact to a greater extent as the wave period increases. The derived threshold flow conditions are converted into threshold shear stress values using existing linear and non-linear models. The performance of the models for various periods of oscillation is then examined in view of the assumption that a unique threshold value exists for a particular grain size. All models suggest that the critical stress of a particular grain is higher under shorter period waves, for the range of grain sizes presented. Using different models for waves, of different periods, appears to remove the effect of wave period upon the data. It is suggested that theories used by the existing models are insufficient to fully describe the interaction of the steady and oscillatory flows during the experiments
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Sediment fluxes and the evolution of a riverine-supplied tectonically-active coastal system: Kyparissiakos Gulf, Ionian Sea (eastern Mediterranean)
No full textVariations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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